Blade fixing for a blade in a gas turbine engine

ABSTRACT

The blade fixing comprises a firtree having a width between two opposite surfaces, each defining a plurality of successive necks separated from one another by arcuate lobes. Each of the opposite surfaces is substantially flat and substantially parallel to the other at the narrowest point of a first one of the necks that is closest to the platform.

TECHNICAL FIELD

The invention relates to an improved fixing for a blade used in a gasturbine engine.

BACKGROUND

Gas turbine engines are often provided with rotors in the compressorand/or the turbine sections thereof where blades are removably connectedto the periphery of the corresponding rotor. While many arrangementshave been suggested in the past to connect the blades to the rotors,room for further improvements still exists.

SUMMARY

In one aspect, the present concept provides a blade fixing for a rotorblade in a gas turbine engine, the blade having a platform with theblade fixing projecting therefrom, the blade fixing comprising a firtreehaving a width between two opposite surfaces, each surface defining aplurality of successive necks separated from one another by arcuatelobes, the firtree having a local minimum width at a narrowest portionof each of said necks and a local maximum width at an apex of each ofsaid lobes, each of the opposite surfaces being substantially flat andsubstantially parallel to the other at the narrowest portion of a firstone of the necks that is closest to the platform.

In another aspect, the present concept provides a firtree for a blade ina gas turbine engine, the firtree generally projecting from a platformof the blade, the firtree comprising at least two pairs of oppositearcuate lobes, each lobe of a pair being on a corresponding side, thelobes generally decreasing in size between successive lobes startingfrom a first of the lobes with reference to the platform, the platformand the first lobe being separated on each of the opposite sides by aneck having a substantially flat section extending parallel to a medialplane of the firtree, the necks uninterruptedly blending to adjacentlobes.

In a further aspect, the present concept provides a fixing for anairfoil blade for a gas turbine engine, the fixing comprising a firtreeprojecting radially from a platform of the airfoil blade and having aplurality of pairs of opposed lobes, the platform and lobes being areasof maximum fixing width separated by corresponding necks, the necksbeing areas of minimum fixing width, one of the necks closest to theplatform having a radial height which is larger than a radial height ofthe other necks.

Further details of these and other aspects will be apparent from thedetailed description and figures included below.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding and to show more clearly how it may becarried into effect, reference will now be made by way of example to theaccompanying figures, in which:

FIG. 1 schematically shows a generic turbofan gas turbine engine toillustrate an example of a general environment in which the improvedblade fixing can be used;

FIG. 2 is a perspective view of an example of a blade with an example ofthe improved blade fixing; and

FIG. 3 is a schematic side view of the bottom part of the blade shown inFIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a gas turbine engine 10 of a typepreferably provided for use in subsonic flight, generally comprising inserial flow communication a fan 12 through which ambient air ispropelled, a multistage compressor 14 for pressurizing the air, acombustor 16 in which the compressed air is mixed with fuel and ignitedfor generating an annular stream of hot combustion gases and a turbinesection 18 for extracting energy from the combustion gases. The engine10 is an example of an environment in which the improved blade fixingcan be used. Blades with the improved blade fixing can be used in thefan 12, the compressor section 14 and/or the turbine section 18.

FIG. 2 illustrates an example of a rotor blade 20 with an example of theimproved blade fixing, which blade fixing comprises a firtree 22. Theblade 20 also comprises a platform 26 under which projects the bladefixing. An airfoil 24 projects over the platform 26 and the firtree 22projects below the platform 26 on the opposite side. The exact shape ofthe platform 26 and of the airfoil 24 illustrated in FIG. 2 is notrelevant in the present specification.

The firtree 22 includes two opposite profiled and continuous sidesurfaces 22 a, 22 b. These surfaces 22 a, 22 b are shaped so as todefine a plurality of opposite pairs of necks 30, 32, 34 separated fromone another by opposite pairs of arcuate lobes 40, 42. They are providedfor connecting the blade 20 to a corresponding profiled slot, calleddisk broach, made in the periphery of a rotor (not shown). A connectorelement (not shown) is also generally provided to prevent to the blade20 from axially moving out of its slot. The firtree 22 prevents theblade 20 from moving radially outward under the intense centrifugalforce when the rotor is rotated at very high speeds. It also transfersother forces between the rotor and the blade 20.

One of the goals in the design of the blade fixing is to minimize theblade weight while still being capable of withstanding the harshconditions in which it is subjected. While providing a blade fixing witha firtree design can effectively reduce weight, the design of the bladefixing must take into account the stress concentrations at some of thelocations, particularly where the width of the firtree 22 is narrowercompared to other locations. These locations correspond to necks of thefirtree 22.

In the improved firtree 22, the two necks 30 closer to the platform 26on the two opposite side surfaces 22 a, 22 b are mutually separated by awidth that remains locally minimal over a given radial height (d)instead of being minimal only at a specific point, in which case theradial height is virtually zero. The radial height is measured in adirection parallel to a medial axis M of the firtree 22. Thecorresponding neck 30 on each side has a portion that is substantiallyflat and extends parallel to the medial plane M of the firtree 22. Theseflat neck portions have an infinite or almost infinite radius ofcurvature and an angle being nil with reference to the medial plane M ofthe firtree 22. The radius of curvature of the narrow portion of thefirst necks is larger than the radius of curvature on either side ofthese portions. There is a local maximum width at an apex of each lobe40, 42.

Also, each of the three necks 30, 32, 34 has a surface uninterruptedlyblending to adjacent surface areas so as to prevent any abrupt junctionanywhere on the side surfaces 22 a, 22 b. The junctions are thus madeusing fillets.

In the illustrated embodiment, the opposite side surfaces 22 a, 22 b aresubstantially symmetrical with reference to the medial plane M of thefirtree 22. Also, as illustrated, the pairs of lobes 40, 42 are two innumber and the lobes 40, 42 decrease in size as a function of thedistance from the platform 26, the maximum width of the lobes 40, 42decreasing towards the extremity of the firtree 22. Necks 32, 34 awayfrom the platform 26 have an arcuate profile at a narrowest portion ofsuch necks 32, 34. Alternative designs are possible as well, especiallyfor the tip 44 of the firtree 22, which illustrated tip 44 is onlyoptional.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that other changes may also be made to theembodiments described without departing from the scope of the inventiondisclosed as defined by the appended claims. For instance, the presentinvention is not limited to a blade as exactly illustrated herein. Theblades can have shapes and proportions that vary in accordance with theneeds. The flat neck portions can be provided anywhere on the firtree.Using a plurality of flat neck portions simultaneously is also possible.Still other modifications which fall within the scope of the presentinvention will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A blade fixing for a rotor blade in a gas turbine engine, the bladehaving a platform with the blade fixing projecting therefrom, the bladefixing comprising a firtree having a width between two oppositesurfaces, each surface defining a plurality of successive necksseparated from one another by arcuate lobes, the firtree having a localminimum width at a narrowest portion of each of said necks and a localmaximum width at an apex of each of said lobes, each of the oppositesurfaces being substantially flat and substantially parallel to theother at the narrowest portion of a first one of the necks that isclosest to the platform.
 2. The blade fixing as defined in claim 1,wherein the remainder of the plurality of successive necks have anarcuate profile at the narrowest portion of the such necks.
 3. The bladefixing as defined in claim 1, wherein each of the surfaces have a radiusof curvature at the narrowest portion of the first neck which is largerthan a radius of curvature on either side of the narrowest portion ofthe first neck.
 4. The blade fixing as defined in claim 1, wherein thenarrowest portion of each of the first necks has a radius of curvaturewhich is larger than a radius of curvature at the narrowest portion ofany other neck of the plurality.
 5. The blade fixing as defined in claim1, wherein the opposite side surfaces are substantially symmetrical withreference to a medial plane of the blade fixing.
 6. The blade fixing asdefined in claim 1, wherein the lobes decrease in width as a function ofa distance between the lobes and the platform.
 7. The blade fixing asdefined in claim 1, wherein opposite portions on the surfacesimmediately adjacent to the platform are part of the first necks.
 8. Afirtree for a blade in a gas turbine engine, the firtree generallyprojecting from a platform of the blade, the firtree comprising at leasttwo pairs of opposite arcuate lobes, each lobe of a pair being on acorresponding side, the lobes generally decreasing in size betweensuccessive lobes starting from a first of the lobes with reference tothe platform, the platform and the first lobe being separated on each ofthe opposite sides by a neck having a substantially flat sectionextending parallel to a medial plane of the firtree, the necksuninterruptedly blending to adjacent lobes.
 9. The firtree as defined inclaim 8, wherein the firtree is substantially symmetrical with referenceto the medial plane.
 10. A fixing for an airfoil blade for a gas turbineengine, the fixing comprising a firtree projecting radially from aplatform of the airfoil blade and having a plurality of pairs of opposedlobes, the platform and lobes being areas of maximum fixing widthseparated by corresponding necks, the necks being areas of minimumfixing width, one of the necks closest to the platform having a radialheight which is larger than a radial height of the other necks.
 11. Thefixing as defined in claim 10, wherein the sides of the firtree haveprofiles that are substantially symmetrical with reference to the medialplane.
 12. The fixing as defined in claim 11, wherein the lobes decreasein size as a function of the distance of the lobes from the platform.